1. Field of the Invention
The present invention relates to a magnetic particle coated material which can be used in a magnetic recording medium and the like, and a method for producing the same.
2. Description of the Related Art
In order to increase magnetic recording density, it is necessary to decrease the size of magnetic particles included in a magnetic layer of a magnetic recording medium. In magnetic recording media widely used as video tapes, computer tapes, disks, and the like, noise decreases along with decreased particle size when the mass of ferromagnetic bodies is the same.
A CuAu type or Cu3Au type ferromagnetic ordered alloy is a material for the magnetic particles desirable for increasing magnetic recording density. The ferromagnetic ordered alloy is known to have high crystal magnetic anisotropy because of strain generated at the time of ordering, and exhibits ferromagnetism even when the size of the magnetic particles is decreased.
Magnetic particles having ferromagnetism are produced by a liquid phase method or a vapor phase method. Particularly, magnetic particles immediately after the production thereof by a liquid phase method have a disordered phase and a face-centered cubic lattice structure.
The face-centered cubic lattice generally exhibits soft magnetism or paramagnetism. Magnetic particles having soft magnetism or paramagnetism are not suitable for use in magnetic recording media. In order to obtain a ferromagnetic ordered alloy having a coercive force of at least 95.5 kA/m (i.e., 1200 Oe) required for a magnetic recording medium, it is necessary to carry out annealing at a temperature not lower than a transformation temperature at which the disordered phase is transformed to the ordered phase. (Refer to, for example, U.S. Pat. Nos. 5,456,986, 6,262,129, 6,302,940 and 6,162,532, and Japanese Patent Application Laid-Open (JP-A) Nos. 2001-93130, 2001-256631 and 2002-157727.)
A spin coater is generally used to form a CuAu type or Cu3Au type magnetic layer on both surfaces of the magnetic recording medium. However, the spin coater cannot form a magnetic layer on one surface of the magnetic recording medium unless the magnetic layer formed on the other surface is dry. Therefore, a method is used in which formation of the magnetic layer on one surface and the other surface of the magnetic recording medium is separately carried out. Namely, first, a material for the magnetic layer is applied onto one surface of a support, and this surface is annealed to form a magnetic layer. Next, the material for the magnetic layer is applied onto the other surface of the support, and this surface is annealed to form a magnetic layer. When annealing is carried out separately to form the magnetic layers, magnetic property of the magnetic layers may vary widely. When the magnetic property widely varies between the magnetic layers, different medium head systems are required so as to correspond to the respective surfaces. Thus, this method is not preferable from practical and industrial standpoints.
Further, in a method in which the material for the magnetic layer is applied onto one surface and subsequently onto the other surface of a support and both the surfaces are annealed at the same time, a drawback arises in that, when the material for the magnetic layer is applied onto the other surface, scratches may be formed on the reverse surface, onto which the material was first applied. Particularly, when the support is formed of an organic material and the magnetic layer is formed by using a spin coater, the support needs to be held on a turn table. Therefore, the aforementioned drawback of scratches becomes significant.
Accordingly, there is a need for a magnetic particle coated material having magnetic layers formed on both surfaces thereof, which magnetic layers have excellent and substantially the same magnetic property (coercive force).